docs: def series_keys(self, flow_id, cache=True):
          '''
          Get an empty dataset with all possible series keys.
  
          Return a pandas DataFrame. Each
          column represents a dimension, each row
          a series key of datasets of 
          the given dataflow.
          '''
          # Check if requested series keys are already cached
          cache_id = 'series_keys_' + flow_id
          if cache_id in self.cache:
              return self.cache[cache_id]
          else:
              # download an empty dataset with all available series keys
              resp = self.data(flow_id, params={'detail': 'serieskeysonly'})
              l = list(s.key for s in resp.data.series)
              df = PD.DataFrame(l, columns=l[0]._fields, dtype='category')
              if cache:
                  self.cache[cache_id] = df
              return df